This invention relates to the reproduction of coloured images and in particular to colour modification carried out in the reproduction process.
It is known to carry out such modification electronically. Thus, a coloured original is scanned through a number of colour filters by a photoelectric device to produce electrical signals representing the colour components of the original. These signals are then modified as required before being used to modulate a light source exposing a photosensitive sheet to produce separation negatives constituting modified representations of the colour components of the original. These negatives are used in the preparation of the printing plates for cylinders for the different colour components, from which multi-colour prints are made.
The colour modification may be of the kind which provides colour correction for imperfections in the reproduction process, for example to compensate for the fact that the colour filters and the printing inks do not provide ideal complementary colour responses.
Alternatively, the colour modification may be of an editorial character, for example to enhance the appearance of flowers in the image or to enhance the flesh colour of a face portrayed in the coloured image.
An example of such a system is described in U.S. Pat. No. 2,799,722, which discloses a method of recognizing a particular "colour" in a picture by its trichromatic coordinates and of operating switches which, when a colour is thus recognized, substitute for the actual trichromatic scanner signals, signals with predetermined desired values. In such a system, the trichromatic signals define the "colour" in such a manner that its hue, saturation and brightness are all fixed.
In commonly assigned British Pat. No. 1294191 there is disclosed a method of colour identification which, instead of responding to a set of three coordinates, responds to two coordinates of "colour space". The same hue-plus-saturation combination was identified at all brightness levels.
The methods described in these two specifications will operate satisfactorily but have not been greatly used; this is probably due to their inflexibility.